1. Field of the Invention
Embodiments disclosed herein relate generally to invert emulsion wellbore fluids. In particular, embodiments disclosed herein relate to invert emulsion fluid having a high internal phase concentration.
2. Background Art
During the drilling of a wellbore, various fluids are typically used in the well for a variety of functions. The fluids may be circulated through a drill pipe and drill bit into the wellbore, and then may subsequently flow upward through wellbore to the surface. During this circulation, the drilling fluid may act to remove drill cuttings from the bottom of the hole to the surface, to suspend cuttings and weighting material when circulation is interrupted, to control subsurface pressures, to maintain the integrity of the wellbore until the well section is cased and cemented, to isolate the fluids from the formation by providing sufficient hydrostatic pressure to prevent the ingress of formation fluids into the wellbore, to cool and lubricate the drill string and bit, and/or to maximize penetration rate.
In most rotary drilling procedures the drilling fluid takes the form of a “mud,” i.e., a liquid having solids suspended therein. The solids function to impart desired rheological properties to the drilling fluid and also to increase the density thereof in order to provide a suitable hydrostatic pressure at the bottom of the well. The drilling mud may be either a water-based or an oil-based mud.
Many wells, especially in oil fields in shale formations (having water sensitivity) and/or deep-water/subsea environments, are drilled with synthetic/oil-based muds or drilling fluids. Because of the extremely high cost of intervention and high production rates, these wells require reliable completion techniques that prevent sand production and maximizes productivity throughout the entire life of the well. One such technique is open-hole gravel packing.
There are two principal techniques used for gravel packing open holes: (1) the alternate path technique and (2) the water packing technique. The latter uses low-viscosity fluids, such as completion brines to carry the gravel from the surface and deposit it into the annulus between a sand-control screen and the wellbore. The alternate path technique, on the other hand, utilizes viscous carrier fluids; therefore the packing mechanisms of these two techniques are significantly different. The alternate path technique allows bypassing of any bridges that may form in the annulus, caused by for example high leakoff into the formation due to filtercake erosion, or exceeding the fracturing pressure, or shale-sloughing/shale-swelling or localized formation collapse on the sand control screens.
In unconsolidated formations, sand control measures are implemented to stabilize formation sand. Common practice for controlling sand displacement includes placement of a gravel pack to hold formation sand in place. The gravel pack is typically deposited around a screen. The gravel pack filters the sand while still allowing formation fluid to flow through the gravel, the screen and a production pipe.
Most of the recently discovered deep-water fields contain a high fraction of shales, which are water-sensitive, although many have been gravel packed with water-based fluids. A very large fraction of them have been completed with viscous fluids using the alternate path technique. Viscoelastic surfactant (VES) solutions have been the most widely used carrier fluid in open hole gravel packing with the alternate path technique due to their low formation and gravel pack damage characteristics, their low drawdown requirements, their capability of incorporating filtercake cleanup chemicals into the carrier fluid, and their low friction pressures.
In wells drilled with synthetic or oil-based muds (often the case for high shale fractions and/or deep water wells), three main approaches have been used for gravel packing. A first approach involves displacement of the entire wellbore to water-based fluids at the end of drilling the reservoir section, and subsequently running the sand control screens into the open hole, setting the packer and gravel packing with a water based fluid. However, as experienced by several operators, the problem with this approach is that exposure of reactive shales to water-based fluids for prolonged time periods can cause shale collapse or swelling which effectively reduces the wellbore diameter and makes it impossible to install sand control screens to the target depth (see SPE 89815, SPE 90758) or shale dispersion into the carrier fluid during gravel packing which can have a significant impact on well productivity (Ali et al.—Petroleum Engineer International, March 1999). The success of this approach is therefore heavily dependent on the reactivity of the shales.
One approach subsequently practiced involved installation of a pre-drilled (perforated) liner in oil-based mud, then displacement of the entire wellbore to water based fluids, subsequent installation of the sand control screens to target depth and finally gravel packing with a water based fluid. This approach solved the problem of inability to run the screens to target depth, since shale collapse would occur onto the pre-drilled liner, and the space inside the predrilled liner would be substantially free of shales, allowing the screens to be installed to target depth. The problems with this approach were two fold. First, it involved two trips (one for predrilled liner installation and another for screen installation), which is costly, particularly in deep water where rig costs are high. Secondly, a smaller size screen had to be installed into the wellbore, which in some cases can limit production rates, and thus increase the costs.
A more recent approach that has been practiced heavily in two deepwater Angola developments (see SPE 90758 and SPE 107297) and an oilfield in Azerbaijan (see SPE 98146) involved conditioning of the oil based mud by passing the mud through shaker screens of a certain size (to prevent plugging of sand screens), then running the sand screens in conditioned oil based mud, and subsequently displacing the wellbore to water based fluids and proceeding with gravel packing with a water based carrier fluid (U.S. Pat. Nos. 6,883,608 and 7,373,978). This approach has been successfully used in more than 75 wells to date by two major operators, in conjunction with the alternate path (shunt-tube) screens.
While the above-described methods may be adequate for installing sand screens and gravel packing in many wells formed in reactive shale-containing formations, there exists a continuing need for improvements in the completion fluids used during such methods.